Copying Visuals in a Scene

I would like to create a (shallow) copy of a Visual in a Scene, along the lines of:

```cpp
ScenePtr scene = CreateSomeScene();
VisualPtr source_visual = scene->CreateVisual("source");

// Make source_visual nice and shiny ...

// Create a visual named target
// target_visual should receive the geometries of source_visual,
// but not copy the child nodes, which would open up problems about naming etc.
VisualPtr target_visual = scene->CopyVisualByName("source", "target");

```

I have looked for alternatives in the newest reference, but have found none. I'm willing to contribute if someone points me to where to begin.

Start by locating the Scene and Visual APIs represented by CreateVisual and the proposed CopyVisualByName, then trace the relevant rendering-engine implementations. Done means a shallow visual copy transfers the source geometries to the target while excluding child nodes, with behavior covered by the project's existing tests.

Written by the indexing model from the issue text.